Diisocyanate polyaddition products dispersed in polyethers or polyesters are already known. According to the teaching given in German Auslegeschrift No. 1,168,075, diisocyanates are reacted with bifunctional primary alcohols in a dispersing medium consisting of a polyether or polyester having a molecular weight of from 500 to 3000 containing at least two, exclusively secondary, hydroxyl groups in the molecule. According to German Auslegeschrift No. 1,260,142, compounds containing isocyanate and amino groups are made to undergo a polyaddition reaction in situ in a polypropylene glycol ether as dispersing agent. The dispersions of polyurethanes, polyureas or polyhydrazodicarbonamides in polyvalent, high molecular weight hydroxyl compounds obtained by the above mentioned processes are recommended as thickeners for the textile or dye industry because of their high viscosities, even at low solid contents. Thus, a 10 or 20% dispersion of polyhydrazodicarbonamide in a polypropylene glycol ether obtained according to German Auslegeschrift 1,260,142, for example, has a viscosity of over 10,000 (200,000) cP at 25.degree. C. This amounts to more than 10 (200) times the viscosity of the pure dispersing agent. When attempts are made to prepare a 40% dispersion, the reaction mixture solidifies before polyaddition has been completed. The high viscosities which occur at even relatively low solid contents seriously restrict the use of the products because, in many fields of application they cannot be dosed with the usual dosing apparatus. For producing polyurethane foams, for example, a purpose for which such dispersions have been recommended according to an earlier proposal by the present Applicants, the viscosities of the starting materials must be below 2500 cP when the conventional high pressure machines are employed.
According to another earlier proposal, substantially anhydrous, relatively low viscosity dispersions of polyureas and/or polyhydrazodicarbonamides in polyethers having hydroxyl groups can be obtained if the polyaddition reaction is carried out continuously in continuous flow mixers. The process has the disadvantage of requiring a relatively complicated technique of dosing and mixing which is uneconomic for customary production rates. Moreover, in some cases high solids concentrations may give rise to considerable difficulties in the removal of the heat of reaction.